Estimating the star formation rate at 1 kpc scales in nearby galaxies.

Abstract

Using combinations of H alpha, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, Sigma(SFR), at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically motivated IR spectral-energy-distribution-based approach to account for possible contributions to 24 mu m emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24 mu m term in hybrid (UV+IR or H alpha+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies, but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24 mu m emission. We compare a large suite of Sigma(SFR) estimates and find that above Sigma(SFR) similar to 10(-3)M(circle dot) yr(-1) kpc(-2) the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that when focusing on a part of a galaxy dominated by a single stellar population, the intrinsic uncertainty in H alpha- and FUV-based SFRs is similar to 0.3 and similar to 0.5 dex.